The Marketplace Growth Team is constantly analyzing data and looking for new ways to help you earn more revenue. The Growth Team recently examined the WordPress category and uncovered a gem: multilingual themes are outperforming single-language themes. In fact, multilingual themes have on average 3 times more sales than non-multilingual WordPress themes. This is a remarkable discovery and a testament to the global reach of the internet. To help you create the best multilingual theme possible, we’ve partnered with OnTheGoSystems, the creators the WordPress Multilingual Plugin, WPML. WPML is a commercial product, normally costing $79. You are invited to receive a complimentary developer account for WPML, so that you can make your themes and plugins multilingual-ready. To apply, visit their Go Global Program. When completing your Go Global Program application, be sure to enter the link to your Envato profile in the designated field.

To help you get started, OnTheGoSystems has agreed to lead two free developer Q&A sessions. These sessions will focus on helping you make your themes and plugins multilingual using the WPML plugin. The first session will be on July 9th at 10 am EST. This session will focus on making your WordPress themes WPML compatible. The second session will be on July 10th at 10 am EST and will focus on making your plugins WPML compatible.

Or if you look at a different way, if a theme has multiple languages included, that may simply imply that the author made a bigger effort than one who made a theme without extra languages. In other words, if you include languages, you probably included other cool features too + worked on the site longer, which would ultimately = more sales.

I kind of agree with Cubell, but there is also this http://s04.flagcounter.com/more/ceGa/ (stats from one of popular WP themes, not mine, hope it’s OK to post it, link was in theme description) at least 30% of visitors – and probably more! – are from nations that do not use English as official language!

Cubell said
Or if you look at a different way, if a theme has multiple languages included, that may simply imply that the author made a bigger effort than one who made a theme without extra languages. In other words, if you include languages, you probably included other cool features too + worked on the site longer, which would ultimately = more sales.
Sometimes correlation does not imply causation.

I originally attributed this to effort. But during a conversation with the CEO of iCanLocalize, he informed me that developers of themes that had become WPML-compatible reported an increase of at least 30% in sales. The increase came after they added Multilingual support. Thanks for the feedback.